Belt Tension Calculator
The Belt Tension Calculator converts a plucked belt’s audio frequency into tension, so you can set both belts identically without guessing.
Free, runs in your browser, no registration.
Why belt tension comes first
Section titled “Why belt tension comes first”Belt tension is upstream of every motion calibration on the machine. Get it wrong and:
- Too loose — ringing that input shaping can’t fully hide, layer shifts, backlash, dimensional inaccuracy, poor repeatability
- Too tight — motor bearing wear, stepper skipping, premature belt failure, and on some frames, visible deflection
Critically, input shaping tuned at the wrong belt tension becomes wrong the moment you retension. Do belts first, then input shaping.
The method
Section titled “The method”Belts under tension behave like guitar strings — the frequency they ring at is a direct function of tension, length, and mass.
- Measure the free span — the unsupported length of belt between the two nearest pulleys or idlers. Not the total belt length. This is the measurement people get wrong.
- Pluck it and record with a frequency analyser. Free apps: Spectroid (Android), Sound Analyzer / AudioTools (iOS), or the Klipper-adjacent
plot_belt_frequencytooling. Any spectrum analyser works. - Read the dominant frequency in Hz.
- Enter frequency, span length, and belt type into the calculator.
Target values
Section titled “Target values”For GT2 6 mm belt, the common target is around 110 Hz on a 150 mm free span, giving roughly 6–8 kg of tension.
Because frequency depends on span length, the number is meaningless without the span. A 300 mm span at the same tension rings roughly an octave lower. Always calculate for your actual measured span rather than copying a Hz figure from a forum.
For CoreXY machines, what matters most is that both belts match. A 5 Hz difference between A and B introduces skew that no software correction fixes properly. Get them within a few Hz of each other, then worry about the absolute value.
Practical notes
Section titled “Practical notes”- Measure cold. A machine that’s been printing has thermal expansion in the frame.
- Pluck near the middle of the free span, gently. You’re looking for a clear tone, not a bang.
- Take three readings and average. Plucking technique varies more than the belt does.
- Recheck after the first few hours of printing on a new build — belts settle and stretch.
- Recheck every few months on any machine. Belts are consumables.
Symptoms that point at belt tension
Section titled “Symptoms that point at belt tension”| Symptom | Likely cause |
|---|---|
| Ringing that input shaping doesn’t fully fix | Belts too loose |
| Layer shifts under fast moves | Belts too loose, or skipping |
| Round holes coming out oval | Belts uneven (CoreXY) |
| Squares printing as parallelograms | Belts uneven — see skew correction |
| Dimensional drift over a long print | Belts loose or slipping |
| Grinding noise from steppers | Belts too tight |
Common mistakes
Section titled “Common mistakes”- Measuring total belt length instead of free span. The calculator needs the unsupported section between pulleys.
- Tuning input shaping before belts. You’ll redo it.
- Over-tightening “to be safe.” Too tight is genuinely damaging — it eats motor bearings and stretches belts permanently.
- Setting the two CoreXY belts by feel. Human fingers cannot reliably match tension. That’s the whole point of measuring.
- Never rechecking. Belt tension drifts. It’s maintenance, not a one-time setup.
Where this sits in tuning
Section titled “Where this sits in tuning”Motion chain: belt tension → input shaping → acceleration limits → skew correction
Run this in parallel with the extrusion chain. They’re independent until you get to high acceleration, where extrusion rate smoothing connects them.
Related tools
Section titled “Related tools”- Input Shaping Calculator — do this after belts
- Skew Correction Calculator
- Run Current Calculator — stepper current, the other half of reliable motion
- All Calculators
Verified values
Section titled “Verified values”Belt tension targets and free-span measurements for my Voron 2.4, Mercury One.1, and K2 Plus are documented in the Minimal 3DP config packs.